1. Technical Field
The invention relates generally to optical metrology and more particularly to the measurement of the width of narrow lines projecting from or recessed in a medium; it finds a particularly important application in the measurement of the width of lines obtained by photo-lithography during the manufacture of integrated circuits and then makes dimensional monitoring of such circuits possible.
2. Prior Art
In this particular field, there exists a need for checking the width of lines obtained by photolithography using a photosensitive resin. Optical methods have already been used, working with visible light. But a problem is encountered: many thin layered materials are transparent to visible light. This is particularly true for the photosensitive resins used in the manufacture of integrated circuits. The transparency results in optical resonance, wave channelling, interference and diffraction optical problems which can only be corrected accurately for small thicknesses not exceeding about 0.5 .mu.m. This limitation is all the more troublesome in that at the present time resin layers are used whose thickness exceeds 1 .mu.m.
Furthermore, most of the existing methods have insufficient repeatability and linearity.